Heretofore, in the case that workpieces made of resin or the like are joined together, vibration welding has been carried out in which a vibratory force is applied to the workpieces, whereby parts to be welded of the workpieces are heated and joined. In vibration welding, there is no need for adhesives, and further, after the welding process, there is no need for heat drying or the like. Thus, vibration welding is suitable for joining together interior members or components of an automobile.
For example, in Japanese Laid-Open Patent Publication No. 2007-091024, a technique is disclosed in which an air bag chute is attached to an instrument panel of an automobile by vibration welding. In this case, so that the instrument panel can be broken smoothly when the airbag is deployed, a tear line, which is a breakable portion (a cutout portion, i.e., a groove-shaped thin portion), is disposed on the instrument panel. The instrument panel and the airbag chute are joined by vibration welding while sandwiching the cutout portion therebetween.